ceph: remove special ack vs commit behavior

- ask for a commit reply instead of an ack reply in
  __ceph_pool_perm_get()
- don't ask for both ack and commit replies in ceph_sync_write()
- since just only one reply is requested now, i_unsafe_writes list
  will always be empty -- kill ceph_sync_write_wait() and go back to
  a standard ->evict_inode()

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>